A tandem seal device wherein mechanical seals are arranged as two stages in an axially same direction is effective to a high-pressure seal, because a pressure load at a sliding face can be reduced by dividing fluid pressure in the two stages. Further, even in case that leakage is occurred at a seal (a primary seal) of a sealed fluid side (an internal side), a seal of atmosphere side (a secondary seal) becomes backup so that fluid leakage can be prevented certainly by two sets of seals. Thus, the tandem seal device is used as a shaft seal device for an apparatus wherein sealing object is liquefied gas and the like, such as a pump used in, for example, a petroleum plant.
In the mechanical seal device used as a secondary seal device of the tandem seal device for such pump, particularly in the mechanical seal device used in dry contact condition in normal using condition, sliding property is very important.
Conventionally, in order to improve the sliding property of the mechanical seal device like this, for example, a sliding material wherein lubricant is immersion (for example, porous carbon, etc.) is used, or a method for making non-contact so as to act a levitation force between sliding faces by dynamic pressure generated by forming a groove on a sliding face of a sliding material are often applied. Further, it is suggested that a method for improving sliding property under dry contact condition so as to reducing load by lowering contact face pressure of sliding faces by introducing a primary seal side pressure via a through hole to whole circumferential groove formed on the sliding face (for example, refer to Japanese Patent Laid Open No. 2006-83889 (Patent Document 1) and Japanese Patent Laid Open No. 2006-83893 (Patent Document 2)).    Patent Document 1: Japanese Patent Laid Open No. 2006-83889    Patent Document 2: Japanese Patent Laid Open No. 2006-83893